Backscatter is an important error source in laser gyros. Backscatter may be understood as a retroreflection of a portion of traveling beams in a laser gyro from scatter points located along the path of the beams and typically found on the surfaces of any solid optical elements within the path. Backscatter results in the coupling of the counter-propagating beams in the gyro and is the primary source of the lock-in problem for two-frequency gyros. Lock-in results in the non-linear variation in the beat frequency for rotation of the laser gyro within range of slow rotation rates. Some schemes to prevent lock-in have concentrated on the backscatter problem. For instance, in U.S. Pat. No. 4,152,071 to Podgorsky, the mirrors are moved under control of a feedback system to a position which minimizes backscatter. In U.S. Pat. No. 4,281,930 to Hutchings, the mirrors forming the laser gyro path are phase dithered to cause the beams to scan the face of the mirrors and thus periodically vary the phase of the scatter at each mirror to reduce lock-in. In U.S. Pat. No. 3,533,014 to Coccoli et al., the coupling of the counter-propagating beams is reduced by oscillating in a predetermined way one of the mirrors in a shear mode.
The main problem with all of these schemes is that variations in the scatter level still remain. Since these variations cannot be predicted, they cannot be compensated. For circularly polarized, or four-frequency, laser gyros these variations then result in the drift of the output signal which is used to indicate rotation rate of the laser gyro.